This invention relates to flame retarded resin compositions that are safe and impose minimal environmental loads.
Due to having excellent mechanical, electrical and thermal properties, molded parts comprising aromatic polycarbonate resins have been commonly used in engineering plastics in a wide variety of applications, including business machines, electric and electronic devices, automotive parts, and in building fields components. To overcome the drawbacks of aromatic polycarbonate resins in regard to their processablity and modability, a number of polymer blends with other thermoplastic resins, such as polystyrene resins, acrylonitrile-butadiene-styrene (ABS) resins and polyester resins have been developed. Among others, polymer alloys comprising an ABS resin have been widely used in the automobile, business machine and electric and electronic fields.
In current applications, including for business machines, electronic appliances and the like, it is strongly desired to have the resin materials flame retardant. To meet this requirement, numerous flame retarding techniques have been proposed for aromatic polycarbonate resins and polymer alloys with other thermoplastic resins.
As disclosed in U.S. Pat. No. 6,043,310, a common practice is to use an organic halide flame retardant containing bromine in combination with a flame retardant aid such as antimony trioxide. Resin compositions of this formulation are made flame retardant to a relatively large extent, but raise environmental problems in giving off harmful or toxic substances upon disposal by incineration. In addition, the production of molded parts from such resin compositions has the disadvantage that, if thermally decomposed, the flame retardant releases hydrogen halide which can cause corrosion of the molds used in the manufacturing process or detract from the physical properties of molded resin parts themselves. For this reason, efforts have been made to achieve flame retardancy without resorting to organic halide compounds containing bromine.
For example, attempts have been made to use oligomeric phosphate esters in combination with polytetrafluoroethylene, which has a fibrillating ability. Aromatic bisphosphates have characteristics such as low volatility, high temperature stability and low tendency to exude the polymer. Aromatic bisphosphates are used as a flame retardant or plasticizer for imparting fire retardancy, temperature stability and good moldability to thermoplastic resin. Also, aromatic bisphosphates show resistance to temperatures up to 300° C., which is necessary for the processing of some engineering plastics.
However, the long-term heat and hydrolytic stability of a polycarbonate (PC)/ABS blend flame retarded with aromatic bisphosphates is not always satisfactory. Furthermore, aromatic phosphates tend to decrease the dimensional stability of the resin at elevated temperatures.
By contrast, silicone resins have high heat resistance, do not emit toxic gases upon combustion, and are safe in themselves. Several types of silicone resins have been proposed as a flame retardant for polycarbonate resins. U.S. Pat. Nos. 5,955,542, 6,001,921, and 6,184,312, and European Patent Application No. 1,094,093, 1,272,565, and 1,293,551 all describe flame retarded resin compositions containing silicone resins comprising di- and trifunctional siloxane units, having a relatively high molecular weight and bearing phenyl groups. Allegedly, these silicone resins, which have branched structures, offer improved heat resistance, and the silicone resins bearing phenyl groups exert flame retardant effects by forming an incombustible silicon-carbon (Si—C) ceramic layer through mutual coupling of aromatic rings on the surface of the resin to which it is added.
Also, U.S. Pat. Nos. 6,326,425 and 6,509,421 describe a thermoplastic resin composition comprising an aromatic polycarbonate resin and a methyl-phenyl or isopropyl-phenyl silicone oligomer.
PCT Patent Application No. WO 99/28387 describes a flame retardant polycarbonate resin composition comprising, as essential components, an aromatic polycarbonate resin, a branched organic silicone and an alkali metal salt of perfluoroalkanesulfonic acid.
European Patent No. 1,277,799 describes a flame retardant polycarbonate resin composition comprising branched methyl-phenyl siloxane in combination with potassium perfluorobutane sulfonate or the potassium salt of diphenylsulfonic acid. Similarly, the use of a branched methyl-phenyl siloxane, either in combination with polytetrafluroethylene or sodium trichlorobenzenesulfonate (U.S. Pat. No. 6,534,576) or in combination with sodium chloride (European Patent Application EP 1,302,510) has also been described.
The use of linear poly(methylsiloxanes) or poly(methylphenylsiloxanes) in polycarbonate compositions is described in U.S. Pat. Nos. 6,541,548, 6,451,906 and 6,454,969.
PCT Patent Application No. WO 02/46288 describes a flame retarded polycarbonate resin composition comprising a poly(methylphenylsiloxane) in combination with bisphenol A bis(diphenyl phosphate). PCT Patent Application WO No. 02/081566 and European Patent Application No. 1,342,753 describe similar compositions but in combination with potassium perfluorobutane sulphonate or diphenylsulfon-3 sulfonate.
The use of cyclic octaphenylcyclotetrasiloxane is described in European Patent Application No. 1,278,798 and in U.S. Pat. No. 6,541,548.
Copolymers of linear polysiloxanes with polycarbonate resin, which provide flame retardant performance, are described in U.S. Pat. Nos. 6,323,280 and 6,576,706 and in PCT Patent Application Nos. WO 03/008501 and WO 03/42305.
By using transmission electron microscopy, it was shown in European Patent Application No. 1,272,565 that in molded parts siloxane moieties are mostly distributed near the surface of the part. It is believed that when these resin compositions are burned, organopolysiloxane molecules or an organopolysiloxane and a resin component are joined through oxidative decomposition and crosslinking of akoxy or organooxy groups, to form a network structure which is fixed in proximity to the burning portion, thus exerting flame retardant effects.